1. Field of the Invention
The present invention relates to a method for forming a porous PTFE layer made of a porous polytetrafluoro ethylene (PTFE) film, a porous PTFE layer being obtained by the forming method, and a molded product including the porous PTFE layer.
2. Description of the Related Art
A porous PTFE film has an excellent mechanical strength property and is suitable for applications such as a membrane filter and an insulating tape to be used for an electric wire and a cable. Moreover, it is proposed that the porous PTFE film is used as a gas transmission film of a fuel battery having a type such as a solid polymer electrolyte or a gas diffusion electrode (see Japanese Patent Application Laid-Open Publication No. 2004-063200 and Japanese Patent Application Laid-Open Publication No. 2005-235519, etc.).
In general, such a porous PTFE film is manufactured by stretching a PTFE film obtained in a calendering process (extrusion processed substance) to improve a porosity and a strength of the film and by orientating a PTFE fibril in a stretching direction. The porosity of a film to be obtained depends on a size of a gap existing between fibrils.
A micropore to be formed in a film by the above stretching is closed in the case in which an elasticity (extensibility) of the film is recovered after the stretching, thereby preventing the porosity of the film from being held. Consequently, after the stretching process, the PTFE film is heated to a temperature that is higher than a crystalline melting point (327° C.) of the PTFE and that is lower than a thermal decomposition temperature (370° C.) of the PTFE, and a PTFE polymer is sintered to lower an elasticity of the film and to maintain a size of a micropore and a porosity.
However, in the case in which the film is contracted and retracted during the above sintering, a wrinkle is easily formed on the film. To suppress this problem, a special means is required. For instance, as described in Japanese Unexamined Patent Application Publication No 11-511707, in the case in which a uniaxially stretched film is prepared, the uniaxially stretched film is heated and sintered while dimensions are fixed by applying a tension in a stretching direction.
On the other hand, it is said to be a problem for research and development to satisfactorily form a laminated structure capable of displaying desired functionality for a molded product such as a tube made of a PTFE film.
For instance, Japanese Patent Application Laid-Open Publication No. 09-123302 discloses a composite tube having a structure in which a burned or unburned porous PTFE layer (outer layer) is laminated and bonded on a peripheral surface of a thermoplastic resin tube (inner layer). As a method to be used for an interlayer bonding, for instance, the inner layer and the outer layer are heat-fused in an integrating manner by a heating treatment (preferably to a temperature equivalent to or higher than a melting point of the PTFE), or an adhesive layer is disposed between the inner layer and the outer layer (paragraph [0013] of the document).
A method for manufacturing a porous multilayer hollow fiber disclosed in Japanese Patent Application Laid-Open Publication No. 2005-329405 is characterized by forming a convex and a concave on a peripheral surface of a porous stretched PTFE tube (support layer), wrapping the tube in a porous stretched resin sheet (filtering layer), applying a load simultaneously with or after the wrapping in order to bond the porous stretched PTFE tube and the porous stretched resin sheet, and sintering the porous stretched PTFE tube and the porous stretched resin sheet in an integrating manner (Claim 1). Patent document 5 describes that the formation of a convex and a concave can prevent the tube and the sheet from being dislocated, and the load can prevent the sheet from floating, thereby improving an adhesive property of the tube and the sheet (paragraph [0039] of the document). Moreover, the porous stretched PTFE tube and the porous stretched resin sheet are sintered at a temperature equivalent to or higher than a melting point of the tube and the sheet (for instance, 350° C. in Embodiment 1), thereby firmly fusing the tube and the sheet in an integrating manner (paragraph [0040] of the document).
Japanese Patent Application Laid-Open Publication No. 11-506987 discloses a method for manufacturing a porous composite material in which a burned porous PTFE layer that is not stretched and not expanded is formed on a stretched expanded PTFE film by spray coating a fluid dispersion of unburned PTFE particles to the stretched expanded PTFE film and by heating the film at a high temperature. In detail, Embodiment 1 describes a manufacturing method in which a mild steel sheet is wrapped in the stretched expanded PTFE film, a fluid dispersion of unburned PTFE particles is spray-coated to the stretched expanded PTFE film, and the stretched expanded PTFE film is heated at 100° C. for 2 hours, at 280° C. for 0.5 hours, and 350° C. for 2 hours.
In a method for forming a PTFE film layer described in the above reference documents Japanese Patent Application Laid-Open Publication No. 09-123302, No. 2005-329405 and No. 11-506987, the PTFE film layer is heated at a temperature equivalent to or higher than a melting point of the PTFE in order to bond the PTFE film layer (in the case in which an unburned PTFE film is used, the PTFE film is burned during the heating treatment as a result). However, such a heating treatment at a high temperature causes a micropore of a porous PTFE to be easily damaged, thereby possibly changing a pore size or closing the micropore by melting.
A size of a micropore of a PTFE film is changed depending on a rapid cooling operation or a slow cooling operation carried out after the heating treatment at a temperature equivalent to or higher than a melting point of the PTFE as well as a degree of stretching (stretching can be carried out before wrapping a support body, and stretching can also be carried out while wrapping a support body). A delicate adjustment is required to obtain a desired size of a micropore finally.
Due to the above reasons, in a conventional method, it is complicated and difficult to properly adjust properties such as a gas transmission rate of a PTFE film layer to be obtained. Consequently, it was impossible to implement a method for manufacturing a PTFE film layer having a specified gas transmission rate that is required for applications such as a gas transmission film of a fuel battery.